A data communication technology making use of a "packet" is widely employed in a data communication network, wherein the packet is a sequence of binary digits including data, call control signals and possibly address which are arranged in a specific format. Perhaps the best known and most widely used protocol standard of a data communication is X.25 protocol. The X.25 specifies an interface between a host system and a packet-switched network.
With the improvement in transmission technology and switching facilities, a new packet communication scheme, "frame relay", has been becoming highlighted and won popularity. The communication scheme using the frame relay, principally based on the X.25, eliminated as many overhead of the X.25 as possible. Thus, the frame relay can be viewed as a streamlined version of the X.25.
As an Integrated Services Digital Network (ISDN) accommodates the frame relay to a large degree, a capability of performing a packet switching for the frame relay is regarded essential in an ISDN switching system.
Since the ISDN switch accommodates a packet switching as well as a circuit switching, in practice, the packet switch can be regarded as sub-functions of the ISDN switch.
A skeleton of the ISDN switch, or the packet switch, is shown in FIG. 1.
The packet switch incorporates itself into major subsystems such as an access switching subsystem (ASS) 100, an interconnection network subsystem (INS) 110 and a central control subsystem (CCS) 120.
The ASS 100 interconnects itself with subscribers on one side and the INS 110 on the other side. The ASS 100 performs call processing, call flow control, time switching functions, and the like.
The INS 22, connected to the ASS 100, is designed for performing space switching, network synchronization and the like.
The CCS 120 supervises and controls overall functions performed in each subsystem in the packet switch.
In the ASS 100, a plurality of access switching subsystems for ISDN subscriber (ASS-I) 101 to 103 are incorporated. The ASS-I provides interfaces by using the ISDN standard interfaces such as I430, I441 and I451. An ISDN subscriber is able to gain access to the packet switch and finally reach another ISDN subscriber with the help of these ISDN standard interfaces and protocols.
An access switching subsystem for packet (ASS-P) 104 designed for handling packets is also included in the ASS 100. All the packets exchanged between subscribers pass and are handled by the ASS-P 104. The ASS-P 104 is not directly connected to subscribers but is connected to the INS 110.
Meanwhile, FIG. 2 provides a closer look at the ASS 100.
A basic rate subscriber interface block (BSI) 201, a primary rate subscriber interface block (PSI) 202 and a basic access rate multiplexing interface block (BAMI) 203 provide different types of interfaces to provide services for various kinds of communication services from ISDN subscribers.
An ISDN subscriber access processor (ISAP) 204 is employed in order to control functions of the BSI 201, the PSI 202 and the BAMI 203.
An inter-processor communication (IPC) network 205 is designed for communications between processors in the ASS-I 101.
An access switching processor-ISDN (ASP-I) 206 is designed for controlling operations occurring within the ASS-I 101. A time switch processor (TSP) 207 controls the operation of a time switch (TSW) 208.
The TSW 208 performs a time slot interchange as a typical time switch does. The TSW 208 constitutes a typical T-S-T switching structure together with another TSW and a space switch (SSW) (not shown).
A detailed inner structure of the ASS-P 104 is described in FIG. 3.
The ASS-P 104 presents a layered structure including packet handling modules (PHMs) 301 to 303, packet layer control processors (PLCPs) 305 to 307 and an access switching processor for packet (ASS-P) 309.
Herein, the PHMs 301 to 303 handle the X.25 and an X.75 protocol. The PHMs 301 to 303 are classified into a PHM-B for handling a B-channel packets based also on the X.25 protocol, a PHM-D for handling D-channel packets based on the X.25 protocol and a PHM-P for providing inter-working services between the ISDN and a public switched packet data network (PSPDN) based on the X.75 protocol.
The PLCPs 305 to 307 perform call processing for packets, and handle routing information.
The ASP-P 309 controls connection and disconnection between the ASS-P 104 and subscriber modules such as the BSI 201, the PSI 202 and the BAMI 203.
A time switch processor (TSP) 310 controls the operation of a time switch (TSW) 311.
The PLCPs 305 to 307 communicate with the PHMs 301 to 303 via a packet bus (P-bus) 304; and communicate with the ASP-P 309 and the TSP 310 by using an inter-processor communication (IPC) network 308.
The above-described conventional packet switch, however, presents some disadvantages.
A packet switching service can be blocked in case of a malfunction or a breakdown of the ASS-P 104 since all the packet data is processed in the ASS-P 104.
In addition, the conventional packet switch leaves something to be desired. In case that a sending subscriber and a receiving subscriber happen to be connected to the same ASS-I, all the packets to be exchanged between the subscribers must pass the ASS-P 104. Time for processing packets is rather longer when the sending subscriber and the receiving subscriber are connected to the same ASS-I than connected to different ASS-Is. In other words, the path the packets travel in the packet switch is ASS-I--SSW (in the INS 110)--ASS-P--SSW (in the INS 110)--ASS-I. In this case, it is desired to have a scheme that the packet data is processed only at the ASS-I, not passing through the SSW.